A large portion of the data storage in today's computers is done on magnetic media such as magnetic disks. Data is presented to a computer by huge quantities of bits (ones and zeroes) and stored on magnetic disks where each bit is represented by an induced magnetic field. In order to read the value of any given bit, a sensor able to detect changes in a magnetic field is required.
To this end, a sensor that changes electrical resistance in response to a magnetic field, called a magnetoresistive (MR) sensor, is employed. Most sensors utilize the anisotropic magnetoresistive (AMR) effect where a read element resistance varies in proportion to the square of the cosine of the angle between the magnetization in the read element and the direction of a sense current flowing through the read element. Data is read by the sensor from magnetic transitions recorded in the media. The magnetic field, resulting from a transition, causes a change in the direction of the magnetization in the read element. The new magnetization direction changes the resistance of the read element with a corresponding change in the sense current or voltage.
Newer sensors, which are more sensitive to smaller recorded transitions on higher density media, are starting to become more commonly used. These sensors use a larger form of magnetoresistance called the giant magnetoresistance (GMR) effect. The GMR effect produces a magnetoresistance that, for selected combinations of materials is greater in magnitude than that of the AMR effect. The GMR effect occurs in multilayer thin films of alternating ferromagnetic and nonferromagnetic metals. The resistance of a GMR film changes according to the cosine of angle between the magnetization of the ferromagnetic (FM) layers.
A subset of the GMR devices is the spin valve in which two ferromagnetic layers, a "free" layer and a "pinned" layer, are used as explained in B. Dieny, et al. "Giant Magnetoresistance in Soft Ferromagnetic Multilayers", Physical Review B., Vol. 43, No. 1, Jan. 1, 1991, pp. 1297-1300 and Dieny, et al. U.S. Pat. No. 5,206,590. When the magnetization in the two layers are aligned, the resistance is at a minimum. When the magnetizations are antialigned, the resistance is at a maximum. The resistance varies as the cosine of the angle between the magnetizations and is independent of the direction of current flow. The magnetization of the pinned layer is held in place by depositing it next to a layer of antiferromagnetic (AFM) material with a resulting exchange coupling of the two layers. The free layer magnetization is free to rotate in response to the field from the disk. In this way, the magnetization swings between being parallel (low resistance state) to anti-parallel (high resistance state) as the head flies over recorded magnetic transitions on the disk. The resulting change in electrical resistance arising from the GMR effect is sensed and the magnetic information on the disk is transformed into electrical signals. Commonly used metallic AFM materials are platinum manganese (PtMn), iron manganese (FeMn), iridium manganese (IrMn), nickel manganese (NiMn), and nickel oxide (NiO).
Unfortunately, the current incarnations of giant magnetoresistive (GMR) sensors using the spin valve effect have a significant problem which is directly related to the FM/AFM (or pinned/pinning) bilayer of the GMR sensor. When subjected to either high temperature annealing or long term high current density operation, interlayer atomic diffusion at the bilayer interface occurs and causes operational instability of the GMR sensor due to the degradation of both the pinning field and the pinning angle of the bilayer. This thermal degradation is a serious problem since thermal annealings of up to 250.degree. C. are required in the process of device fabrication for curing organic insulators coated on metal surfaces of the GMR sensors. Besides, the GMR sensor itself is expected to undergo long-term operation under a high current density. A solution, which would form an FM/AFM bilayers for GMR sensor with improved thermal stability, has long been sought but has eluded those skilled in the art.